The present invention relates to an apparatus for measuring a thickness of a film formed on an object in accordance with polarization analysis.
A chromium oxide film is formed on a plated surface of a tin free steel (TFS) plate or a nickel-plated steel plate. A thickness of a chromium oxide film greatly influences adhesion of an anti-corrosive paint to be coated on the steel plate. Control of the thickness of the chromium oxide film is one of the most important control factors in the manufacture of steel plates.
A thickness of an oil film applied by an electrostatic oil coater to a surface of a steel plate on an electric tin plating line (ETL), a TFS plate or a tin plate for export use is also one of the most important control factors for guaranteeing the quality of products.
A thickness of a chromium oxide film or an oil film is from a multiple of ten to several hundred angstroms. A highly precise measuring machine is required to measure such a fine thickness on the manufacturing line.
For this purpose, a film thickness measurement method by polarization analysis is proposed as one of the methods of measuring very fine thicknesses. According to this method, light from a light source is filtered through a monochromatic filter to obtain a monochromatic light component. The monochromatic component is incident on a first polarizer to obtain a linearly polarized light component. This linearly polarized light component is then elliptically polarized by a phase compensator. An elliptically polarized light component is then incident on an object at a predetermined angle of incidence. The polarizer and the phase compensator are arranged to linearly polarize light reflected by the object. The reflected light then passes through a second polarizer and is detected by a photodetector. When the actual thickness of the object coincides with a preset value, a phase shift acting on the incident light in accordance with an optical constant of the film cancels the phase shift caused by the phase compensator. Therefore, when the polarization directions of the first and second polarizers are directed such that the amount of light detected by the photodetector becomes zero, an unknown film thickness can be measured.
According to this method, however, the phase shift caused by the phase compensator must be adjusted every time a film thickness changes so as to set the amount of light detected by the photodetector to be zero (extinguished). When an object is conveyed at high speed, accurate on-line film thickness measurement cannot be performed in practice.